linux-kernel - Re: [PATCH 0/2] vhost: improve livepatch switching for heavily loaded vhost worker kthreads

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Message-ID: <20230131163832.z46ihurbmjcwuvck@treble>
Date:   Tue, 31 Jan 2023 08:38:32 -0800
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Petr Mladek <pmladek@...e.com>,
        Joe Lawrence <joe.lawrence@...hat.com>, kvm@...r.kernel.org,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, netdev@...r.kernel.org,
        Jiri Kosina <jikos@...nel.org>, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        "Seth Forshee (DigitalOcean)" <sforshee@...italocean.com>,
        live-patching@...r.kernel.org, Miroslav Benes <mbenes@...e.cz>
Subject: Re: [PATCH 0/2] vhost: improve livepatch switching for heavily
 loaded vhost worker kthreads

On Tue, Jan 31, 2023 at 10:22:09AM +0000, Mark Rutland wrote:
> > Hm, it might be nice if our out-of-line static call implementation would
> > automatically do a static key check as part of static_call_cond() for
> > NULL-type static calls.
> > 
> > But the best answer is probably to just add inline static calls to
> > arm64.  Is the lack of objtool the only thing blocking that?
> 
> The major issues were branch range limitations (and needing the linker to add
> PLTs),

Does the compiler do the right thing (e.g., force PLT) if the branch
target is outside the translation unit?  I'm wondering if we could for
example use objtool to help enforce such rules at the call site.

> and painful instruction patching requirements (e.g. the architecture's
> "CMODX" rules for Concurrent MODification and eXecution of instructions). We
> went with the static key scheme above because that was what our assembled code
> generation would devolve to anyway.
> 
> If we knew each call-site would only call a particular function or skip the
> call, then we could do better (and would probably need something like objtool
> to NOP that out at compile time), but since we don't know the callee at build
> time we can't ensure we have a PLT in range when necessary.

Unfortunately most static calls have multiple destinations.  And most
don't have the option of being NULL.

-- 
Josh